The present invention relates generally to steam generating systems for producing process steam and/or steam for operating a steam turbine, and more particularly to a new and improved method and apparatus for the recovery of additional heat from the stack gas of the steam generating system.
The heat recovery system has notable application in cogeneration plants of the type conventionally employing a gas turbine for generating electricity, coupled to a heat recovery steam boiler system for generating steam from the high temperature exhaust gas. In such a plant, it is desirable to regulate the boiler system exhaust gas temperature within a preestablished desired temperature range to minimize exhaust gas heat loss while maintaining the exhaust gas above a temperature at which undesirable condensation occurs. This type of plant is described for example in U.S. Pat. No. 4,572,110, "Combined Heat Recovery and Emission Control System", issued Feb. 25, 1986.
It is a principal object of the present invention to provide in a steam generating system a new and improved method and apparatus for recovering heat from the exhaust gas in such a way as to increase the overall efficiency of the plant without undesirable condensation within the exhaust gas.
It is another object of the present invention to provide in a cogeneration plant of the type described a new and improved method and apparatus for regulating the stack temperature within a preestablished desired temperature range for efficient operation of the cogenerating plant.
It is a further object of the present invention to provide in a steam generating system a new and improved method and apparatus for employing an economizer of the system for controlling the stack temperature while minimizing or preventing internal and external corrosion of the economizer tubes and minimizing or preventing undesirable feed water steaming within the economizer.
These and other objects are accomplished with the present invention, by providing a new and improved method and apparatus for preheating the system make-up water through a heat exchange with the system feedwater, and thereby decreasing the auxiliary steam requirement for the feedwater deaerator while increasing the exhaust gas heat transfer to the feedwater in the economizer.
In accordance with the present invention, a cogeneration plant is provided with a combined heat recovery and makeup water heating system in which the feedwater from the deaerator heater is placed in heat exchange relation with the makeup water, thereby increasing the temperature of the makeup water delivered to the deaerator and decreasing the temperature of the feedwater delivered to the economizer or similar coil in the stack. In this arrangement, the makeup water is deaerated and, after mixing with the feedwater in the deaerated heater, is delivered to the economizer in a condition that will not cause internal corrosion of the economizer tubes.
With the makeup water heated to a temperature approaching that of the feedwater, the fluid entering the deaerator heater is at a higher average temperature and thus requires less auxiliary steam from the steam generator in order to sustain the design operating conditions. The net effect is an increase in plant efficiency by indirectly transferring the low temperature stack gas heat to the make-up water. Therefore, the steam savings associated with heating the deaerator can be delivered to the load.
In the preferred embodiment, the invention further includes a controller responsive to the temperature of the exhaust gases in the stack downstream of the evaporator for adjusting the amount of heat transferred from the feedwater to the makeup water. This adjustment is intended primarily to avoid the stack gas temperature from dropping below the condensation point of materials in the gas stream that can corrode the exterior of the economizer tubes, e.g. sulfuric acid. Preferably, the controller operates a three-way valve through which makeup water is delivered to the heat exchanger or bypassed around the heat exchanger in proportions that are determined by the control algorithm.
The present invention thus improves the steam generating efficiency of the plant, while avoiding both internal and external corrosion of the economizer. The advantages of the present invention in terms of steam plant performance improvement, are substantially proportional to the ratio of makeup water flow to feedwater flow. Thus, the invention is of particularly noteworthy advantage when used in conjunction with a cogeneration plant having steam injection to a gas turbine in the primary cycle.